Ligands reactions of coordinated

A considerable volume of interesting work has been published in this area. 

Interesting work in this area continues, and a number of useful synthetic applications of cobalt(III) complexes have been described. 

The reactions described to this point are either substitution reactions or oxidation-reduction reactions. Other reactions are primarily those of the ligands in these reactions, coordination to the metal changes the ligand properties sufficiently to change the rate of a reaction or to make possible a reaction that would otherwise not take place. Such reactions are important for many different types of compounds and many different conditions. Chapter 14 describes such reactions for organometallic compounds and Chapter 16 describes some reactions important in biochemistry. In this chapter, we describe only a few examples of these reactions the interested reader can find many more examples in the references cited. 

As usual, it is easier to study reactions of inert compounds, such as those of Co(lll), Cr(IIl), Pt(ll), and Pt(IV), in which the products remain complexed to the metal and can be isolated for more complete study. However, useful catalysis requires that the products be easily separated from the catalyst, so relatively rapid dissociation from the metal is a desirable feature. Although many of the reactions described here do not have this capability, those with biological significance do, and chemists studying ligand reactions for synthetic purposes try to incorporate it into their reactions. 

Rate and equilibrium constant data, including substituent and isotope effects, for the reaction of [Pt(bpy)2]2+ with hydroxide, are all consistent with, and interpreted in terms of, reversible addition of the hydroxide to the coordinated 2,2 -bipyridyl (397). Equilibrium constants for addition of hydroxide to a series of platinum(II)-diimine cations [Pt(diimine)2]2+, the diimines being 2,2 -bipyridyl, 2,2 -bipyrazine, 3,3 -bipyridazine, and 2,2 -bipyrimidine, suggest that hydroxide adds at the 6 position of the coordinated ligand (398). Support for this covalent hydration mechanism for hydroxide attack at coordinated diimines comes from crystal structure determinations of binuclear mixed valence copper(I)/copper(II) complexes of 2-hydroxylated 1,10-phenanthroline and 2,2 -bipyridyl (399). 

Nucleophilic attack at the carbonyl-carbon of 4,5-diazafluoren-9-one (dzf, 12) coordinated to ruthenium has been demonstrated, and a mechanism outlined, for reaction of [Ru(bpy)2(dzf)]2+ with l,8-diazabicyclo[5,4,0] undec-7-ene (dbu)13, which gives a tris-2,2 -bipyridyl derivative (400). 

Whereas heating [Pt(terpy)(N3)]+ in the gas phase gives dinitrogen, heating in solution in acetonitrile or benzonitrile gives the tetrazolato complexes 14 (R = Me, Ph), presumably by nucleophilic attack by RCN at coordinated azide and subsequent closure of the N4C ring (401). 

New NMR information on the state of coordinated 2-formylglycinate in [Co(en)2(formgly)], showing it to be in hydrate plus enol form rather than in aldehyde form, has led to a new theory for the mechanism of the reaction of coordinated formylglycinate with penicillamine (402). 

Hydrolysis of coordinated ligands is a special case of nucleophilic attack. Two examples involving inorganic ligands have already been given in Section II. A on aquation of cobalt(III) complexes. Many further examples will be found in the following Section VII.B on catalysis of hydrolysis of organic substrates by metal ions and complexes. 

It has been known for decades that a range of reactions occurs that involve chemistry of the ligands and in which metal-ligand bond cleavage is not involved. We can regard these as reactions of coordinated ligands. These early and deceptively simple studies provide fine examples of chemical detective work. One of the earliest studies probed the preparation of a H20—Co(III) species from a O2CO—Co(III) precursor, a reaction which was seen to 

An example of a self-assembly reaction of a monomeric complex and ligand, forming a macrocyclic tetranuclear complex. 

Not only can this type of reaction occur for this and a range of other coordinated oxyanions, but also what is effectively the reverse reaction can occur, where the reactive species is a HO —Co(III) complex, which as the 180-labelled form retains the label essentially exclusively in the product, proving that it is a reaction between the nucleophilic coordinated hydroxide and an electrophile, as exemplified for the following well-known reaction that produces coordinated nitrite ion (6.41). In this case, the formation of the thermodynamically 

Other reactions of simple anions that may occur in the absence of coordination can also be observed to occur for the complexed form, with the rate of reaction usually changed significantly as a result of complexation. This is anticipated, since a coordinated ion is bonded directly to a highly-charged metal ion, which must influence the electron distribution in the bound molecule and hence its reactivity. Two well-known examples where the product is ammonia occur through either reduction of nitrite with zinc/acid or oxidation of thiocyanate with peroxide. The former example is exemplified in (6.42) below. 

Transition metal complexes can promote reactions by organizing and binding substrates. We have already seen this in terms of metal-directed reactions. Another important function is the supply of a coordinated nucleophile for the reaction, which is incorporated in the product. We have already seen a coordinated nucleophile at work in the reaction discussed above of Co— OH with NO+ nucleophiles, which are electron-rich entities, are best represented in coordination chemistry by coordinated hydroxide ion, formed by proton loss from a water molecule this is a common ligand in metal complexes. Normally, water dissociates only to a very limited extent, via 

Such reactions have already cropped up in Sections 5.7.1. and 5.7.3, for both acid-catalyzed aquation and mercury(II)-assisted aquation are reactions in which the coordinated ligand plays a central role. Other manifestations of ligand reactivity are dealt with in this section. 

The kinetics of stereoselective deuteration of malonate hydrogens in bis(malonato)-cobalt(III) complexes [Co(mal)2L2] containing L2 = en, pn, N,N -Me2en, phen, c/5 -(NH3)2, or c/5-(py)2 have been monitored. Both acid and base hydrolysis are observed, and there is a reversal of stereoselectivity with solution pH. There are some kinetic differences between the amine ligands on the one hand and py and phen on the other, as competition for OD between malonate and amine is possible, between malonate and py or phen not. The rate law for deuteration of a hydrogens in a-aminocarboxylato complexes of cobalt(III) containing various combinations of glycine, sarcosine, or alanine with ammonia, ethylenediamine, or diaminopropane is simple second order. 

Values of k2 depend on the geometry and charge of the complex, but are little affected by the nature of the amine or ammine ligands. 

An nmr study of the kinetics of proton exchange at cobalt(III)-amine complexes in liquid ammonia has been mentioned in the section on base hydrolysis (Section 5.7.2).  

Substitution Reactions of Inert Metal Complexes—6 and Above 

Electron-transfer and intramolecular redox reactions (related to 82 complexes). The redox behavior of 82 complexes is of particular interest because it can probably provide a foundation for understanding the course of reactions involved in relevant enzymes and catalysts (especially hydrodesulfurization catalysts). Intramolecular redox reactions related to type la 82 ligands can be summarized as follows  

Examples of step 1 are provided by oxidation of =8 and —8R groups (85,132,159,161). 8tep 2 involves reduction of the 8—8 bond to form two sulfido groups (172,173). An example of step 3 is thermal decomposition of Cs2[Mo2(82)6] nH20 to give 82 as the main gaseous product (67) (vide infra), and examples of step 4 include synthesis of 82 complexes from reactions employing elemental sulfur (vide supra and note added in proof). 

Reactions with nucleophiles (with abstraction of neutral sulfur). A characteristic reaction of 8 ligands is abstraction of a sulfur atom by nucleophiles (N) such as PR3, 803 , 8R , CN , and OH , e.g., the reaction (87,105,110,116,125, 130) 

The strongly distorted M0484 cube of the latter species is produced by abstraction of a sulfur atom from each of the four type III ligands. A reasonable mechanism for the reaction of the two type lib ligands would 

Type III bridging 82 ligands are more susceptible to nucleophilic attack with extrusion of a neutral sulfur atom than are type la ligands. This is consistent with a generally lower electron density on the S atoms of type III ligands (bonded to two metal atoms) than of type la ligands (bonded to one metal atom). 

Coordination to the metal changes ligand properties sufficiently to make possible reactions at the ligands that either (a) could not happen with the unbound ligand or (b) could occur without the metal but much more slowly. Reactions at coordinated ligands are a vital aspect of organometallic chemistry (Chapter 14). We will describe a few examples of these reactions within coordination chemistry. 

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This book is a highly readable introduction to the reactions of coordinated ligands. Bridging the gap between the traditional fields, this text presents the basic concepts of ligand reactivity as well as many synthetic applications of these reactions. 

Lewis Acid Catalysis and the Reactions of Coordinated Ligands... 

Minghetti, G., Bonati, F. and Banditelli, G. (1976) Carhene Complexes Of Gold (III) And Reactions Of Coordinated Ligand. Inorganic Chemistry, 15(7), 1718-1720. 

Reactions such as this are electrophilic substitutions, and other electrophiles such as N02 , Cl l3CO, or Cl IO can be substituted on the rings without destroying the complex. Such chemical behavior illustrates the extreme stability of complexes of this type. It is apparent that the nature of the bonding in the chelate rings is as important as is their size. Other reactions of coordinated ligands will be shown in later chapters. 

CH3Mn(CO)4THF has been identified, so the conjecture that THF and DMF behave in a manner that is different from the other solvents is not without some justification. Additional work on the influence of the solvent could yield additional insight regarding the mechanisms of this and other reactions of coordinated ligands. 

J. E. Backvall, Nucleophilic Attack on Coordinated Alkenes , in Reaction of Coordinated Ligands (Ed. P. S. Braterman), Plenum Press, London 1986, pp. 679-731. 

Schrock RR (1986) In Braterman PR (ed) Reactions of coordinated ligands. Plenum,... 

B35. G. Wilkinson, R. D. Gillard and J. A. McCleverty (eds) Comprehensive Coordination Chemistry, Vol. 1. Theory and Background, Pergamon, Oxford, 1987. Chapter 7.1, M. L. Tobe (Substitution) Chapter 7.2, T. J. Meyer and H. Taube (Electron Transfer) Chap. 7.4 D. St. C. Black (Reactions of Coordinated Ligands). 

B41. P. S. Braterman, ed. Reactions of Coordinated Ligands Vol. 1, Plenum 1986. Comprehensive but mainly descriptive Vol. 2, Plenum, 1989 covers coordinated COj, Nj, nitrosyls, O- and N-bound,... 

In connection with our studies of the reactions of coordinated ligands, we have investigated several additional syntheses of iron isonitrile complexes. 

Scheme 9. Preparation of osmium carbonyl/hydride/phosphine or arsine synthetic intermediates via the reactions of coordinated ligands.

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